In recent years, there are various kinds of demands associated with sheet processing of documents output from image forming apparatuses such as a copier, a facsimile, a printer and so forth. Among such demands, in addition to the side stapling, there is a growing demand for saddle stitching in which a plurality of locations of the sheet, generally two locations, are stapled down at a given interval along a centerline dividing the sheet into half. When performing saddle stitching compared with the side stapling, a sheet conveyance distance will vary depending on the size of the sheet, as the sheet is transported to the position at which the saddle stitching is performed.
Related arts associated with the saddle stitching proposed in Japanese Patent Laid-Open Application Publications, No. 2002-128383 and No. 2004-292163, for example, are known. According to Japanese Patent Laid-Open Application Publications, No. 2002-128383 and No. 2004-292163, a rear end portion of a sheaf of paper in a conveyance direction stacked on a stacking mechanism is aligned by a rear-end plate, and subsequently, the rear-end plate retracts from the conveyance path so that the sheaf of paper is transported to a position at which the side stapling or the saddle stitching is performed. When the sheaf of paper reaches the stapling position, the conveyance operation is stopped, and a stapling operation is carried out.
According to the related arts, the end portion of the paper sheaf in the conveyance direction stacked on the stacking mechanism is in contact with the rear-end plate so that the paper sheaf in the sheet conveyance direction is aligned. At this time, the position of the rear-end plate is already fixed. Then, the rear-end plate is moved forward, i.e., away from the fixed position. As may be understood from the related arts, only a single rear-end plate is provided. Consequently, when large size paper such as A3 paper or Double Letter Size (DLT) is stacked on the stacking mechanism, the distance at which the paper sheaf conveyance mechanism carries the sheet so as to perform saddle stitch will be extended. As a result, the center portion of the paper sheaf projects causing the sheet to be misaligned during the transfer of the paper sheaf, or the positioning accuracy of the position of the saddle stitching is deteriorated due to variations in the conveyance amount of the paper sheaf, environment and so forth.